African rainforest : Evolution and Threats

英语. peer reviewed

英语. According to recent data (GLC2000 map), African dense forests cover c. 236 million ha, that is 7.9 % of the total continent area. The Central African forest is the last contiguous block of tropical forest in Africa; it covers an area of c. 203 million ha, whereas the forest cover of West Africa (c. 10 million ha) is highly fragmented. At the fringes of the Congo Basin, forests are replaced by woodlands and shrublands, mixed in some places with croplands. Woodlands and shrublands together cover 27 % of the continent, while grasslands represent 15 % of the land. The largest desert in the world is the Sahara desert that covers an area of c. 900 million ha or 33 % of the African continent. Agricultural activities extend over 12 % of the continent. The area of cropland is more important in West Africa than in Central Africa, respectively 16 % and 7 %. Closely related to agriculture, the deforestation rate is higher in West Africa (1.7 % versus 0.4 %). The other causes of deforestation are logging, mining activities, infrastructure development and fires. The most forested countries of the continent are: Central African Republic (c. 8 million ha), Gabon (c. 21 million ha), Cameroon (c. 21 million ha), Congo (c. 26 million ha) and D. R. Congo (c. 125 million ha). These five countries account for 85 % of the African dense forest. In Central Africa, 45 % of the total forest area is under a logging concession status. As a consequence, logging activities play a major role in the Central African rainforest’s future. According to the ITTO (International Tropical Timber Organization) figures, in 2003 the estimated productions of the five previously mentioned countries were: c. 7.4 million m3 of logs; c. 1.1 million m3 of sawn wood; c. 380,000 m3 of plywood and veneer, that is less than 1 % of the world production. Logging in Central Africa is characterized by its high selectivity. For economic reasons, only a few species are commercialized. Usually only one to two trees are felled per hectare and the cover disturbance is about 10 %. The most felled species can be grouped in two categories: hygrophilous evergreen rain forest species and moist semi-evergreen rain forest species. The first category is mainly composed of azobe Lophira alata, beli Julbernardia pellegriniana, douka Tieghemella africana, kevazingo Guibourtia spp., okoume Aucoumea klaineana, pachyloba Afzelia pachyloba, tali Erythrophleum ivorense. The second category is made up of afromosia Pericopsis elata, agba Prioria balsamifera, ayous Triplochyton scleroxylon, dibetou Lovoa trichiloides, frake Terminalia superba, iroko Milicia excelsa, kossipo Entandrophragma candollei, moabi Baillonella toxisperma, movingui Distemonanthus benthamianus, padouk Pterocarpus soyauxii, sapelli Entandrophragma cylindricum, sipo Entandrophragma utile. The major forest companies in the region are mostly subsidiaries of larger European firms (French, German, etc.) although Asian business is becoming more and more important. Obliged by the recent evolution of the Central African forestry laws, some of these logging companies are conceiving forest management plans. The management methods currently implemented were developed as normative regulations issued under the forest laws in the region. The basis of these methods is a statistical inventory of the resources which allows the determination of the population structures, i.e. the number of stems per diameter class. Considering increment and mortality, rotation and/or minimum felling diameters are adapted to reach a satisfactory reconstitution rate. Unfortunately, undamental knowledge about the species population dynamic parameters is still lacking and finally the precision on the rate of reconstitution after a cutting cycle is jeopardized. Moreover, these methods do not pay any attention to long-term preservation of the logged species, since species could have a satisfactory level of reconstitution after one rotation, but could disappear after a second cycle. For such species specific interventions are needed to improve their natural regeneration capacities. In this paper, examples are given for a 176,000 ha management unit in Cameroon for flagship species: assamela Pericopsis elata, ayous Triplochyton scleroxylon, iroko Milicia excelsa, moabi Baillonella toxisperma, sapelli Entandrophragma cylindricum and tali Erythrophleum ivorense. The obtained results attest the need to implement sylvicultural techniques in order to maintain the production capacity of the African rain forests, this production capacity being the best guarantee for the preservation of their integrity.